1. Field of the Invention
The present invention relates to a phase change optical recording medium for recording and reproducing information by irradiating a laser beam and an optical recording method using this phase change optical recording medium.
2. Description of the Related Art
An optical disk by which information is recorded and reproduced by irradiating a laser beam has a large capacity and a high access speed equivalent to those of a rigid magnetic disk. In addition, the optical disk has medium portability equivalent to that of a floppy disk or a tape medium. Accordingly, the optical disk achieves an excellent performance in storing various information such as images, sounds, and computer data. A phase change optical disk among such optical disks has the following advantages. That is, the phase change optical disk is readily compatible with a read-only optical disk represented by CD-ROM. Also, the phase change optical disk can be easily overwritten by light intensity modulation, and this increases the data transfer rate. Furthermore, in the phase change optical disk, land-groove (L-G) recording proposed as a high-density technology can be realized with a single optical beam. For these reasons, the phase change optical disk is expected to find widespread use in the future as the recording density is improved.
To improve the recording density of the phase change optical disk, it is necessary to decrease the recording mark pitch or the recording mark size. Three methods of decreasing the recording mark pitch are pulse width modulation recording/mark-edge detection, superresolution readout and L-G recording described above. In the land-groove recording method, the depth of a groove is set to about 1/6 of the laser wavelength to reduce crosstalk record data in lands and grooves. A density about twice that of a conventional method in which data is recorded only in lands or grooves can be expected. The pulse width modulation recording method records information in the edge portion of a recording mark, and a density about 1.5 times that of the conventional mark position recording can be expected. The superresolution readout is proposed primarily for ROM media. This method improves the readout resolution by forming a masking layer, whose light transmittance changes in accordance with the intensity of a readout laser beam, on the light source side of a recording pit surface (e.g., Proc. ISOM '95, We-Cl, Fr-Dl). When these high-density recording technologies are used, a current recording density of approximately 10.sup.6 bit/mm.sup.2 is expected increase to 10 to 20 times. However, to further improve the recording density, a new method must be established.
The use of a short-wavelength laser is being studied to decrease the recording mark site. A method of decreasing the recording mark size can be accomplished by forming a recording mark only in the vicinity of the end portion of the laser beam spot and decreasing the recording laser power. Unfortunately, this method results in significantly narrowing the recording power margin as will be described below.
As is well known, in the phase change optical disk, the intensity of the readout signal from a recording mark during readout shows a steep rise and saturation (so-called .GAMMA. characteristic) with respect to the recording power during recording. Assume that the actual temperature of a phase change optical recording layer is T and the melting point of a recording layer material is T.sub.m. Since T&lt;T.sub.m in the entire laser irradiated portion until the recording power reaches the threshold power (P.sub.th), no recording marks are formed. When the power reaches P.sub.th, fine marks begin to formed in a region where T=T.sub.m. When the power exceeds P.sub.th, the region where T.gtoreq.T.sub.m expands to enlarge the recording marks, and the intensity of the readout signal from the recording marks sharply rises accordingly. However, when the region where T.gtoreq.T.sub.m expands to make the recording marks equivalent to the full-width at half maximum (FWHM) of the laser beam spot, the readout signal intensity is saturated without increasing very much even if the recording power is further increased. The reason for this is as follows. Because the intensity profile of a laser beam shows a nearly Gaussian distribution, recording marks are not easily formed in a region exceeding the full-width at half maximum of the recording spot. Additionally, even when excessively large recording marks are formed, these recording marks do not contribute to the readout signal intensity because the intensity distribution of the readout spot is also a Gaussian distribution.
To record data stably in a phase change optical disk, it is desirable for the margin of the recording power be wide. Therefore, a value corresponding to the saturation region of the .GAMMA. characteristic is chosen as the recording power. Also, the .GAMMA. characteristic is preferably as steep as possible because a wide margin of the recording power can be set even at a relatively low laser power. However, as described previously, simply decreasing the recording power for the purpose of decreasing the recording mark size is equivalent to selecting a value corresponding to the rise region of the .GAMMA. characteristic as the recording power. This is impractical because the margin of the recording power is significantly narrowed, and so the readout signal intensity largely varies with a slight power fluctuation.